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You can't change the pKa of CO2, which is 6.3. Any attempt to grow bicarb complexes below pH 7.5 will be problematic due to CO2 bubble formation, which may crack the crystal. What we do in these situations is to soak crystals in a cryo solution at a higher pH for as long as practical, then transfer to another, identical solution with bicarbonate fo li and soak. This can be tricky, because crystals may take a long time to pH equilibrate, crack, or dissolve. At pH 7.5 or above, CO2 formation at chemical equilibrium is minimal. At low pH, you can also consider using acetate as a bicarbonate analog.
Roger Rowlett
On Oct 2, 2011 4:56 PM, "Jacob Keller" <[log in to unmask]> wrote:
> Dear Crystallographers,
>
> I would like to soak my crystals in bicarbonate (a possible
> substrate), but the crystals have grown--and only grow--in pH 5.2-6.0,
> so the bicarb/CO2 will just keep evolving out of the solution and
> reliquishing its hydroxyls until the pH is elevated sufficiently out
> of range. Does anyone have a clever way of getting bicarb into these
> crystals? Grow them under CO2? Transfer them to higher pH, and hope
> for the best?
>
> Jacob Keller